German Auslegeschrift 20 35 619 and U.S. Pat. No. 3,590,065 disclose that mercaptosilanes can be prepared in a single-step reaction by reacting haloorganylsilane compounds suitable for this reaction with thiourea and ammonia. However, this method has the disadvantage that, for the achievement of economically acceptable reaction rates, long reaction periods of more than 24 hours are required. The yields which can be achieved by using this method vary and reach values only between 75 and 80%, based on the reacted amount of the haloorganylsilane compound which is used. Furthermore, this method is burdened with an unavoidable formation of guanidine hydrochloride, the separation of which requires additional equipment, labor and cost.
An attempt was made to improve the method described in U.S. Pat. No. 3,590,065. By adding reaction accelerators to the mixture of reactants, a reduction of the reaction times to about 6 hours was achieved. However, relative to the yields and reaction temperatures to be applied, no improvements were obtained (see U.S. Pat. No. 4,401,826).
Processes for the preparation of mercaptosilanes by reaction of corresponding thioacetosilane compounds with alcohols have not acquired any technical importance because of low yields and limited applicability of this method (see U.S. Pat. No. 3,632,826; 3,565,937; 3,565,935; and German Auslegeschrift 20 38 715).
Processes for the preparation of mercaptosilanes which start, for example, from thiopropionic acid amide silanes and convert the same by hydrogenation under pressure into the corresponding mercaptosilanes have not acquired any technical importance either, because the yields obtained therefrom are unsatisfactory (see European Patent 0 018 094). Likewise, attempts to hydrogenate cyanoalkylsilane compounds in the presence of elemental sulfur or hydrogen sulfide to obtain mercaptosilanes produced no satisfactory results with respect to yields (see U.S. Pat. No. 4,012,403).
It has also been attempted to prepare mercaptosilanes by selective cleavage of special thioethersilanes with the aid of Friedel-Crafts catalysts. This method of preparation, however, is not economically feasible because of the expensive preparation of the corresponding precursors (see German Patent 23 40 886).
The preparation of mercaptosilanes by reacting the corresponding haloorganylsilane compounds with hydrogen sulfide in the presence of ethylenediamine and large amounts of heavy metal sulfides leads to the formation of diverse by-products which are difficult to separate from the target product and lead to a two-phase raw product system, the work-up of which is burdened with the problems described in German Patent 33 46 910 (see also U.S. Pat. No. 3,849,471).
Certain improvements over the method of preparation described in U.S. Pat. No. 3,849,471 were obtained when the reaction of the starting silanes with hydrogen sulfide was not performed in the presence of diamines but in the presence of ammonia, primary, secondary or tertiary amines, and optionally also in the additional presence of polar, protic or aprotic media (see U.S. Pat. No. 4,082,790). This method of preparation has the disadvantage that, in order to achieve the reaction temperatures which are required for the reaction of the reactants, the reactions must be performed in an autoclave. If the reactions are performed in the absence of polar media, uneconomically long reaction periods have to be accepted in order to achieve acceptable rates of reaction. The addition of polar media to the mixture of reactants also results in only minor reductions of the reaction periods; however, because of the increased solubility of the hydrochlorides which are formed as by-products of the reaction, all the difficulties arise which are mentioned in German Patent 33 46 910 and which can be remedied only by means of corresponding costly measures, whereby the process is additionally burdened.
The preparation of mercaptosilanes by reacting hydrogen sulfide with ethylenic unsaturated silanes with the aid of ultraviolet light or in the presence of metals of the eighth side group of the Periodic System leads to small yields of target product; the reaction, moreover, is impaired by the formation of substantial amounts of by-products (see U.S. Pat. No. 3,890,312 and German Auslegeschrift 10 00 817).
British Patent 1,102,251 discloses the reaction of alkali metal hydrogen sulfides with haloalkylsilanes in a methanolic medium to form the corresponding mercaptosilanes. This method of preparation, however, requires extraordinarily long reaction periods in order to achieve high reaction rates, and the yields which are obtained are unsatisfactory. An improvement of the process is achieved with respect to shortening of the reaction period and increasing the yield if the reaction is allowed to proceed while simultaneously introducing hydrogen sulfide. However, the raw product work-up with respect to the removal of the hydrogen sulfide which is dissolved in the reaction mixture up to virtual saturation, and the difficulties arising in the distillative work-up of the reaction product due to continuous precipitation of alkali metal chlorides, remain problematic.